An individual battery cell has a rather low voltage, typically in the range of 1 to 4.2 volts. This low voltage is quite suitable for some purposes, such as small flashlights, watches, handheld calculators and personal radios. However, a single cell is inadequate for uses which have higher voltage and/or current requirements, such as forklifts, golf carts, electric vehicles, electrically started vehicles, and uninterruptable power supply (UPS) systems. For example, automobiles typically require 6 or 12 volts, some diesel powered vehicles require 24 volts, UPS systems require 120 or 240 volts, and some other systems require even higher voltages. The battery cells are connected in series to achieve these higher voltages.
All cells are not identical, and all batteries are not identical. Rather, the particular purity of the materials used, the temperature during construction, and the placement of the plates in a cell cause each cell to be unique. Further, even when cells are constructed with materials from a single batch, constructed at the same time, and constructed with the same tolerances so as to be as closely matched as possible, the subtle differences in these variables cause the cells to become less similar as time passes. As a result, at some point the different cells may have such different states of charge that one or more cells may be fully charged but other cells may have minimal or no charge. When a cell finally reaches the point that it is discharged but the other cells are still at least partially charged, further use of the series-connected cells will cause the discharged cell to be subjected to a reverse polarity voltage, which can cause further deterioration of that cell, overheating, gassing, or even an explosion.
The construction of batteries in a compact, efficient volume dictates that the cells cannot stand isolated from one another. Rather, the cells are placed against each other. When more than two cells are involved, this usually results in some cells being on the outer portion of the battery and some cells being on the inner portion of the battery. The cells on the outside of a battery are able to dissipate heat by conduction to a cooling structure if available, or by convention (forced or natural). However, the cells on the inside of a battery are rather insulated and receive less cooling. An inner cell may be able to dissipate some heat via the top and bottom surfaces of the cell, but these surface areas are rather small and therefore have limited heat dissipation capability. An inner cell also has sides, which have larger surface areas, but these are in contact with other cells. Therefore, in order for an inner cell to dissipate heat via the outer cells the inner cell must be at a higher temperature than the outer cells. Thus, during heat-generating operations, such as charging, the inner cells will be at a higher temperature than the outer cells. Thus, the particular environment in which a cell is used can cause a cell to become more and more different from other, once-similar cells.
Likewise, batteries may be connected in series and/or in parallel, as needed to obtain the desired output voltage and energy storage capacity. Similarly, the assembly of a set of batteries in a compact, efficient volume for a battery pack, such as a battery power supply or an uninterruptible power supply, dictates that the batteries cannot stand isolated from one another. Rather, the batteries are placed in close proximity to or against each other, with some of the batteries being on the inside of the battery pack, and some of the batteries being on the outside of the battery pack. Similarly as with cells, the batteries toward the inside of the battery pack have less heat dissipating capability than the batteries toward the outside of the battery pack. Therefore, in order for an inner battery to dissipate heat via the outer batteries the inner battery must be at a higher temperature than the outer batteries. Thus, during heat-generating operations, such as charging, the inner batteries will be at a higher temperature than the outer batteries. However, batteries on the outside of the battery pack may also be subjected to greater and more rapid extremes in temperature than the batteries which are on the inside of the battery pack and are therefore somewhat insulated from the surrounding environment. Likewise, similar batteries experience different environments. For example, one 12 volt battery may be a year older than another 12 volt battery and may have been subjected to more or fewer charge/discharge cycles, more or fewer deep discharge cycles, higher or lower temperature extremes, etc.
Therefore, it is more likely than not that the temperature, the internal impedance, and the state of charge will be different from battery to battery in a battery pack and will be exaggerated as the batteries undergo aging, temperature cycling, and charging/discharging cycles. Thus, at some point, one of the batteries will reach a state of zero charge when others of the batteries still have substantial charges. Further discharging of the battery pack will cause the battery with zero charge to be subjected to a reverse polarity voltage, with the same consequences for that battery as described above for an individual cell which is reverse charged.
At 90% of full charge, a cell will not readily accept a high charging rate. Therefore, if the charging current is set so as to rapidly charge the weakest cell, the charging current will be too high for a more fully charged cell and damage can be done to the more fully charged cell. However, if the charging current is reduced to prevent damage to the more fully charged cell then the charging process will take a much longer time. For example, if each battery in a battery pack has a full-charge rating of 12 volts and 200 ampere-hours, all batteries but one are fully charged, and this one battery has a state of charge of only 90% of full charge, then 20 ampere-hours of charging current must be applied to that battery to bring it to a full charge. To accomplish this, a 20 amp charge could be applied for 1 hour, or a 40 amp charge could be applied for 30 minutes, or a 160 amp charge applied for 7.5 minutes, etc. However, the fully charged batteries may not accept the 160 amp charging current, or even the 40 amp charging current, without overheating, gassing, or damage. Therefore, to avoid damage to the fully charged batteries during the equalization process, the charging current must be limited to 20 amperes, or less, and the charging time must be extended to 1 hour, or more, to add enough charge to the lesser charged battery to bring it to the same full charge level as the other batteries.
As a result, each cell becomes a unique component, with its own output voltage, energy storage capacity, internal resistance, leakage rate, and maximum charging rates and conditions. Thus, the cells in a battery, and the batteries in a battery pack, perform differently than other cells or batteries, and these differences change as the battery is aged and is used. Therefore, each cell, and each battery, has its own, unique set of charging and discharging parameters.
Thus, the differences in the individual cells, and the differences the individual batteries, and the differences in heat dissipating capability, can easily result in temperature differences of 20 degrees Fahrenheit (11 degrees Celsius).
Electric vehicles, hospitals, aircraft, ships, power production stations, airport towers and radars, telephone central offices and relay stations, radio stations, television stations, and other systems require battery packs, either as a primary power source or as a backup power source, such as in a UPS. In some cases, the battery pack may comprise dozens of batteries connected in series-parallel configurations.
Lithium batteries are particularly sensitive to overcharging. If a Lithium battery is seriously or repeatedly undercharged or overcharged then its lifetime will be greatly shortened. If a lithium battery is overcharged then irreversible dissolution of electrolyte will occur involving oxygen and heat evolution. Likewise, if a lithium battery is over discharged, such as may occur due to undercharging before use, then nickel or cobalt (depending upon the construction of the battery) will be deposited onto the carbon electrode. This is an irreversible chemical reaction which reduces the lifetime of the battery. For example, the charge-discharge cycle lifetime of a lithium battery capable of 1000 charge-discharge cycles may be reduced to as few as 10 charge-discharge cycles. A lithium battery is currently more expensive than even a silver-zinc battery. Thus, undercharging and overcharging can be expensive. Therefore, care must be taken that a lithium battery is neither overcharged nor undercharged in order to avoid this life-cycle shortening and reduce the maintenance costs of systems involving lithium batteries.
In any battery, and in any battery pack, regardless of the type, the likelihood that one or more of the cells will be undercharged or overcharged depends upon the number of cells connected in series. The larger the number of series-connected cells, the greater the likelihood that one or more of the cells will be undercharged or overcharged. Some companies attempt to alleviate this problem by matching the cells in a battery or the batteries in a battery pack. This matching is generally done by measuring the open circuit voltage of each fully charged battery or by measuring the internal resistance of each fully charged battery. However, this procedure is expensive and time-consuming. Further, this procedure does not compensate for differences that arise due to aging or environment.
The problem, undercharging or overcharging, is compounded by series-parallel configurations. One series of cells or batteries may have a higher voltage than another series. The higher voltage series will then supply current to lower voltage series. This reduces the charge on the higher voltage series and increases the charge on the lower voltage series. This can cause over discharging or reverse charging of cells in the higher voltage series, and overcharging of cells in the lower voltage series. In addition to damaging some of the cells, this process also results in the overall configuration having less capacity.
Overcharging a cell causes overheating, loss of electrolyte, and gassing. Further, in the last stages of the charging process, the cell is nearly charged and cannot accept as large a charging current as it did when it was only slightly charged. Thus, a cell can be damaged by overheating, loss of electrolyte, and gassing even when the cell is not yet fully charged. In either case, if the charging process continues without modification then the cell will become seriously damaged. Excessive gas production in a sealed cell also shortens the lifetime of the cell by drying out the separator. In lead-acid cells, overcharging shortens the lifetime of the cell by loss of electrolyte, and also cause ozone production which corrodes the cell and causes other changes in the cell chemistry.
A cell which is at a higher temperature can accept a larger charge current and provide a larger load (discharge) current than a cell which is at a lower temperature. Thus, when cells are connected in series, but are not at the same temperature, one cell may be charged at the optimum rate, given its temperature, while other cells may by undercharged or overcharged, thus leading to damage to the cell and/or reduced capacity for the battery.
Also, the internal impedance will be different from one battery to another. The internal impedance depends upon the state of charge of the battery, the temperature of the battery, the amount of electrolyte present, the amount of water in the electrolyte, and the state (deterioration) of the electrodes. A good battery will have a lower impedance when fully charged and a higher impedance when fully discharged. The more that the charging voltage exceeds the battery voltage, the more the current that will be forced into the battery. If the amount of current forced into the battery exceeds the current that the battery can use for charging then the excess current will cause electrolysis of the battery water, gassing, and heating of the battery. Therefore, when a charging current is applied to a battery pack greater heating will occur in a more fully charged battery than the heating in a lesser-charged battery. The states of charge between different batteries may be somewhat equalized by continuing to apply a charge to the battery pack even though some of the batteries have already been completely charged. However, gassing as well as overheating of these more fully charged batteries may occur. Furthermore, if high a current pulse charging technique is used then the application of a large charging current pulse to a fully charged battery may cause damage to or catastrophic failure of the battery.
Therefore, to maximum the lifetime and capacity of a battery or battery pack, it is necessary to accurately determine the state of charge of individual cells in a battery and then to equalize the charges on the individual cells. Equalization is the process whereby all of the cells or batteries are brought to the same state of charge. Equalization is very important because it prevents the application of a reverse polarity voltage to a battery.
Some examples of procedures for measuring the state of charge or equalizing the charges are shown in the following U.S. Pat. Nos. 3,979,658; 3,980,940; 4,238,721; 4,242,627; 4,562,398; 4,331,911; 5,498,490; 4,502,000; 5,528,122; 5,504,415; 5,594,320; and 5,592,067.